In multi-user communication over linear, dispersive, and noisy channels, the received signal is composed of the sum of several transmitted signals corrupted by inter-symbol interference, inter-user interference, and noise. Examples include TDMA digital cellular systems with multiple transmit/receive antennas, wide-band asynchronous CDMA systems, where inter-user interference is also known as multiple access interference, wide-band transmission over digital subscriber lines (DSL) where inter-user interference takes the form of near-end and far-end crosstalk between adjacent twisted pairs, and in high-density digital magnetic recording where inter-user interference is due to interference from adjacent tracks.
Multi-user detection techniques for multi-input multi-output (MIMO) systems have been shown to offer significant performance advantages over single user detection techniques that treat inter-user interference as additive colored noise and lumps its effects with thermal noise. Recently, it has been shown that the presence of inter-symbol interference in these MIMO systems could enhance overall system capacity, provided that effective multi-user detection techniques are employed.
The optimum maximum likelihood sequence estimation (MLSE) receiver for MIMO channels was developed by S. Verdu, “Minimum Probability of Error for Asynchronous Gaussian Multiple Access Channels,” IEEE Transactions on Information Theory, Jan. 1986, pp. 85-96. However, its exponential complexity increases with the number of users, and channel memory makes its implementation costly for multi-user detection on severe-inter-symbol interference channels.
Two alternative transceiver structures have been recently proposed for MIMO dispersive channels as well. These structures, which are widely used in practice for single-input single-output dispersive channels, are the Discrete Multitone and minimum-mean-square-error decision feedback equalizer (MMSE-DFE). In the latter category, this includes A. Duel-Hallen “Equalizers for Multiple Input Multiple Output Channels and PAN Systems with Cyclostationary Input Sequences,” IEEE Journal on Selected Areas on Communications, Apr. 1992, pp. 630-639; A. Duel-Hallen “A Family of Multiuser Decision-Feedback Detectors for Asynchronous Code Division Multiple Access Channels,” IEEE Transactions on Communications, February/March/April 1995, pp. 421-434; J. Yang et an “Joint Transmitter and Receiver Optimization for Multiple Input Multiple Output Systems with Decision Feedback,” IEEE Transactions on Information Theory, September 1994, pp. 1334-1347; and J. Yang et al “On Joint Transmitter and Receiver Optimization for Multiple Input Multiple Output (MIMO) Transmission Systems,” IEEE Transactions on Communications, December 1994, pp. 3221-3231. Alas, the prior art does not offer a practical MIMO MMSE-DFE receiver with feedforward and feedback FIR filters whose coefficients can be computed in a single computation (i.e., non-iteratively) in real-time under various MIMO detection scenarios.